1. Field of the Invention
The present invention relates to acoustic surface wave devices and, more particularly, to an acoustic surface wave device and method in which an acoustic surface wave propagation path is folded through the use of an acoustic discontinuity in the surface of an acoustic substrate to thereby provide increased time delays and/or bulk wave discrimination.
2. State of the Prior Art
Acoustic surface wave devices have become exceedingly useful in a variety of systems as signal processing elements. For example, acoustic surface wave devices are particularly useful as signal filters and signal delay lines in various frequency ranges because of the flexibility of design characteristics of such devices.
Typical acoustic surface wave delay lines are fabricated on substrates of various materials depending upon the range of frequencies at which the delay line is designed to operate. In the higher frequency ranges on the order of hundreds and megahertz to gigahertz, crystalline materials are typically employed as the acoustic substrate. In the lower frequency ranges on the order of 10 to 30 megahertz, amorphous glasses, fused quartz, or steel strip substrates may be employed. To achieve long delays with acoustic surface wave delay lines, a surface wave is typically propagated along the surface of a moderate or low loss acoustic surface wave propagating substrate material in a single path which is long enough to provide the desired delay. Such single path structures may require a relatively large, one piece substrate for the long delay which may be encountered. Moreover, for delay lines of wide fractional bandwidth, the surface wave mode may overlap bulk wave modes at the optimum frequencies for generating surface waves with interdigital transducers. The surface wave and bulk wave modes usually propagate with different velocities and, in a surface device, the bulk wave modes thus represent spurious energy which may cause large non-linear delays.